The Vav family is a group of oncoproteins with three representatives in vertebrates (Vav, Vav2, and Vav3) and single members in invertebrates. These proteins catalyze the exchange of nucleotides on GTP-binding proteins of the Rho/Rac family, thereby facilitating the transition of these GTPases from their inactive (GDP-bound) to their active (GTP-bound) state. The enzyme activity of Vav proteins is tightly regulated during signal transduction by direct tyrosine phosphorylation. As a consequence, they only become activated when phosphorylated by upstream receptors with intrinsic or associated tyrosine kinase activity. Several lines of evidence demonstrate that the function of Vav proteins is crucial for both developmental and mitogenic processes. Thus, the deletion of vav and vav2 genes by homologous recombination results in defective maturation of lymphocyte lineages and lack of proper antigenic responses. Furthermore, the ectopic expression of gain-of-function mutants of Vav and Vav2 induces high levels of cellular transformation in rodent fibroblasts. Finally, recent results have implicated the Vav pathway in the pathogenic cycle of human lymphotropic viruses such as HIV, HTLV, and gamma-herpesviruses. The long-term objective of our laboratory is to achieve a comprehensive characterization of this protein family at the biochemical, structural, cellular, and organism level. To achieve this objective, the following studies will be carried out. In Aim 1, structural biology and biochemical techniques will be used to visualize the conformational changes underwent by these proteins upon binding to upstream regulators and GTPase substrates. In Aim 2, proteomic and expression cloning approaches will be used to reveal regulatory mechanisms of Vav proteins based on both posttranslational modifications and interactions with other intracellular molecules. In Aim 3, signaling techniques will be utilized to dissect new crosstalk and feedback mechanisms operating in the Vav pathway. In Aim 4, available knockout mice for the three vav family genes will be used to study the role of Vav proteins in cell signaling and physiological processes that, when deregulated, contribute to human disease. Taken together, these studies should provide a unified vision of the modus operandi of Vav proteins during signal transduction and, at the same time, supply valuable information for the design of pharmacological tools that could aid in the manipulation of Vav family-dependent biological processes in the future.